The ability to measure surface temperature quickly and efficiently for both near and distant surfaces without making contact with the surface is key to many scientific, engineering and industrial applications. In industries as diverse as steel, aluminum and integrated circuit manufacturing, it is necessary to control and monitor surface temperatures to absolute accuracies of a few degrees. Contact techniques, though most accurate in principle, are often limited by geometry, physical constraints, and the time it takes for the thermal sensors to equilibrate with the surface of interest, as well as errors due to conductive heat flow.
Other engineering and scientific applications may, in addition, require a simultaneous estimate of the surface emissivity and of the size (projected area) of a distant unresolved point target.
Methods that rely on measuring the optical radiation from a surface and relating it to the surface temperature have been used for many years. Generally known as pyrometric techniques, these methods are based on Planck's law of radiation. The main problem with traditional pyrometric techniques is that they need information about the surface spectral emissivity to ultimately relate the measured optical radiation to the actual surface temperature. This is the case whether the measurements are made in only one or in many optical wavelength passbands. Spectral emissivity data is generally difficult to measure and, even when known to some extent, is likely to deviate from the original measurements due to the processes that the material in question is undergoing, as well as oxidation, aging, etc. In addition, most materials are non-gray, that is, the spectral emissivity of the material varies with wavelength, and it may also vary with time.
Another problem with pyrometric techniques has to do with the necessity for compensating for other background radiation sources in the area of the surface of interest. For the case when one is dealing with surfaces which are highly reflective, that is, having low emissivity, the radiation emitted by background sources will combine with the self-emitted surface radiation. Since only the self-emitted radiation contains information related to the surface temperature, it is necessary to compensate for reflected background radiation.
Techniques based on measuring the optical radiation in a single wavelength band are conceptually the simplest of all. However, in relating the measurements to temperature they lack the well-known leverage gained by measuring radiation at several wavelengths. Techniques that employ two or more bands are further subject to spectral variation in the emissivity.
Those techniques that employ many bands, known as multi-wavelength pyrometry, are of two types. One type approximates the Planck distribution by Wien's law to reduce the problem of determining temperature to a linear estimation problem. By using Wien's approximation, however, the technique is limited in the range of wavelengths where the measurements may be carried out for a given range of surface temperatures.
The second type uses the Planck distribution itself, thus posing the problem of estimating temperature as the non-linear estimation problem it actually is. Here, as in the linear technique above, it is necessary to assume a model functional dependence between the spectral emissivity and wavelength. The surface temperature and the coefficients that ultimately determine the emissivity are determined simultaneously by non-linear least squares estimation procedures that perform a fit to the optical data using the specific model. Earlier methods suffer from several disadvantages. They do not account and compensate for background reflected radiation, and they generally require optical measurements in a large number of sensor bands. It is claimed that the statistics involved with fitting a large number of data points using a model with a relatively small number of degrees of freedom will compensate for measurement error. While this might indeed be the case, the practical problem of measuring at a large number of wavelength bands, along with the complexity of non-linear search techniques, makes these procedures too complicated and too slow for many industrial requirements where speed and simplicity are of the essence. An implementation of one such technique also appears to be very sensitive to the presence of noise in the measurements.